Field Line Resonance Research Articles

Global Pc5 event during 29–31 October 2003 magnetic storm

On 30–31 October 2003 magnetic observatories over the whole Earth surface observed a very strong and enduring long-period ULF activity during the recovery phase of the magnetic storm, which had begun a day before, at 0611 UT on October 29th, 2003. Amplitude of oscillations in the Pc5 frequency range was as high as 150 nT at mid-latitude stations. We use magnetic data from INTERMAGNET database, IMAGE magnetometer network, and GOES-10,-12 satellites to study this event. According to IMAGE data, Pc5 polarization pattern along a meridian does not correspond to that expected for field line resonance model. Latitude profile of the horizontal components has its maximum at auroral latitudes. At medium latitudes, Pc5 were observed at all local time sectors simultaneously with larger amplitude at prenoon or noon hours. Boundary layer instability is suggested as a probable source of the global magnetospheric wave activity. Possible consequences of large amplitude oscillations for charged particle acceleration owing to various wave-particle interaction mechanisms are discussed.

Solar eclipse effect on geomagnetic induction parameters

Abstract. The 11 August 1999 total solar eclipse had been studied using a large array of stations in Central Europe (Bencze et al., 2005). According to the result of this study, the amplitudes of the field line resonance (FLR)-type pulsations decreased in and around the dark spot by about a factor of 2, and this decrease moved with the velocity of the dark spot in the same direction. This decrease was interpreted as a switch-off of the FLR-type pulsations, due to a change in the eigenperiod of the field line as a consequence of a change in the charged particle distribution along the field line. An effect was also found in the phase of the (magnetic or electric) perpendicular components. At the Nagycenk (NCK) observatory lying in the zone of totality, both magnetic and electric records were available. The magnetotelluric (MT) sounding curve computed by the usual method for the eclipse interval (08:00-14:00 UT) fits the previously known standard curve extremely well. During the eclipse, however, impedance values in the FLR period range were highly scattered. The scatter remained as long as the eclipse lasted. Coherence values between magnetic and electric components decreased significantly. In contrast, an earlier similar switch-off of the FLR-type activity on the same day did not cause a similar scatter, in spite of a comparably low coherence. Thus, the lack of FLR-type activity disturbed the usual MT connection between the magnetic and electric components during the eclipse. The induction vector (tipper), especially its real part, shows a clear effect of the eclipse in the FLR period range (24-29 s), too. Both at NCK and at Bad Bergzabern (BBZ, westernmost station and longest FLR period), a definite decrease in the real tipper was ascertained during the totality. The average direction of the tipper did not change. Concerning both parameters, a random effect cannot fully explain the observed phenomena. The scatter of the EM induction parameters is most likely due to the switch-off of the FLR activity. The possibility of such an effect should be considered in induction studies. Pilipenko and Fedotov (1993) supposed an opposite effect and emphasised lower quality data, if resulting from FLR-type pulsations, while we claim high quality data just from such an activity.

Behaviour of torsional Alfven waves and field line resonance on rotating magnetars

Torsional Alfven waves are likely excited with bursts in rotating magnetars. These waves are probably propagated through corotating atmospheres toward a vacuum exterior. We have studied the physical effects of the azimuthal wavenumber and the characteristic height of the plasma medium on wave transmission. In this work, explicit calculations were carried out based on the three-layered cylindrical model. We found that the coupling strength between the internal shear and the external Alfven modes is drastically enhanced, when resonance occurs in the corotating plasma cavity. The spatial structure of the electromagnetic fields in the resonance cavity is also investigated when Alfven waves exhibit resonance.

Evolution and characteristics of global Pc5 ULF waves during a high solar wind speed interval

We present an interval of extremely long‐lasting narrow‐band Pc5 pulsations during the recovery phase of a large geomagnetic storm. These pulsations occurred continuously for many hours and were observed throughout the magnetosphere and in the dusk‐sector ionosphere. The subject of this paper is the favorable radial alignment of the Cluster, Polar, and geosynchronous satellites in the dusk sector during a 3‐hour subset of this interval that allows extensive analysis of the global nature of the pulsations and the tracing of their energy transfer from the solar wind to the ground. Virtually monochromatic large‐amplitude pulsations were observed by the CANOPUS magnetometer chain at dusk for several hours, during which the Cluster spacecraft constellation traversed the dusk magnetopause. The solar wind conditions were very steady, the solar wind speed was fast, and time series analysis of the solar wind dynamic pressure shows no significant power concentrated in the Pc5 band. The pulsations are observed in both geosynchronous electron and magnetic field data over a wide range of local times while Cluster is in the vicinity of the magnetopause providing clear evidence of boundary oscillations with the same periodicity as the ground and geosynchronous pulsations. Furthermore, the Polar spacecraft crossed the equatorial dusk magnetosphere outside of geosynchronous orbit (L ∼ 6–9) and observed significant electric and magnetic perturbations around the same quasi‐stable central frequency (1.4–1.6 mHz). The Poynting vector observed by the Polar spacecraft associated with these pulsations has strong field‐aligned oscillations, as expected for standing Alfvén waves, as well as a nonzero azimuthal component, indicating a downtail component to the energy propagation. In the ionosphere, ground‐based magnetometers observed signatures characteristic of a field‐line resonance, and HF radars observed flows as a direct consequence of the energy input. We conclude that the most likely explanations is that magnetopause oscillations couple energy to field lines close to the location of Polar, setting up standing Alfvén waves along the resonant field lines which are then also observed in the ionosphere. In the absence of monochromatic dynamic pressure variations in the solar wind, this event is a potential example where discrete frequency pulsations in the magnetosphere result from the excitation of a magnetospheric waveguide mode, perhaps excited via the Kelvin‐Helmoltz instability or via overreflection at the duskside magnetopause.

Three‐dimensional global hybrid simulation of dayside dynamics associated with the quasi‐parallel bow shock

A three‐dimensional global‐scale hybrid simulation is carried out, for the first time, for dynamics of the dayside bow shock‐magnetosphere system associated with the quasi‐parallel bow shock. A case with IMF along the Sun‐Earth line is examined in detail. First, the foreshock waves and the associated shock reformation process are investigated. In particular, the generation and structure of diamagnetic cavities, with a decrease in the magnetic field and density, in the foreshock of the quasi‐parallel shock are discussed. Second, the interaction of the foreshock‐originated pressure pulses with the dayside magnetosphere is simulated. The diamagnetic cavities that are generated in the turbulent foreshock due to the ion beam plasma interaction are found to lead to strong surface perturbations at the magnetopause. Third, the coupling between the pressure pulses and the magnetosphere is studied. The compressional waves are found to mode convert to shear Alfvén waves and kinetic Alfvén waves through the Alfvén resonance process in nonuniform plasmas. The shear Alfvén waves lead to field line resonance, which corresponds to the fundamental odd resonance wave number, and produce field‐aligned currents in the dipole magnetospheric field.

Excitation of field line resonances by sources outside the magnetosphere

Abstract. Field line resonances are thought to be excited by sources either at the magnetopause or outside it. Recent observations suggest that they may be associated with coherent oscillations or pressure pulses in the solar wind. In either case the excitation mechanism can be understood by considering the incidence of a harmonic wave on the magnetopause from outside the magnetosphere. Calculations are performed in a plane stratified model that consists of (i) a magnetosheath region streaming tailward at uniform velocity (ii) a sharp boundary representing the magnetopause, (iii) a magnetosphere region in which the Alfvén speed increases monotonically with distance from the magnetopause. The structure implies the existence of a propagating region within the magnetopause bounded by a reflection level or turning point. Beyond this is a region in which waves are evanescent and a resonance level. The reflection and transmission of harmonic waves incident from the magnetosheath is considered in this model. It is shown that, in most cases, because of the mismatch between the magnetosphere and the magnetopause, the wave is reflected from the magnetopause with little penetration. At critical frequencies corresponding to the natural frequencies of the cavity formed between the magnetopause and turning point the signal excites the cavity and may leak evanescently to the resonance. The calculation includes the effect of the counter-streaming magnetosheath and magnetosphere plasmas on the wave. This can lead to amplification or attenuation. The nature of the processes that lead to transmission of the wave from magnetosheath to resonance are considered by synthesising the signal from plane wave spectra. A number of mechanisms for exciting cavity modes are reviewed and the relationship of the calculations to these mechanisms are discussed. Observations needed to discriminate between the mechanisms are specified.

The computation of field-line resonance frequencies in general geometries: a tool for improving the understanding of magnetospheric configurations

Until recently, the calculation of field-line resonance (FLR) frequencies from magnetic field data, generated by magnetospheric models such as BATSRUS and Tsyganenko (T01), was restricted to orthogonal coordinate systems. With this restriction, only dipolar and axisymmetric configurations are admissible. The matter of addressing more general configurations such as non-axisymmetric stretched and twisted magnetic field topologies requires the use of a non-orthogonal coordinate system. This coordinate system can be constrained by defining the magnetic field as the product of Euler potentials, B = ∇α i × ∇α j , and imposing the condition, V B = 0, everywhere. As a consequence, the coordinates, α i , must satisfy the partial differential equation, B·∇α i = 0. In other words, α i must be constant along magnetic field lines. Upon solving this differential equation implicitly using known magnetic field intensities, the metric tensor for the resulting basis can be computed. The elements of this tensor can be substituted directly into the eigenvalue problem for general coordinate systems written in covariant notation. The equation for FLR modes has been developed for arbitrary incompressible magnetospheric conditions and has been specialized to the case where spatial variations are constrained along the magnetic field. The result is a fourth-order system of ordinary differential equations, which can be evaluated numerically, provided that the variation of the metric coefficients as a function of the distance along field lines is sufficiently smooth. The eigenvalue problem is solved at several latitudes for a broad range of magnetospheric conditions.

Alfvénic field line resonances in arbitrary magnetic field topology

In this paper we present a general covariant–contravariant formalism suitable for describing standing shear Alfvén waves in a general magnetic field topology. We define a non-orthogonal field-aligned coordinate system based on Euler potentials and compute the metric tensor coefficients for these coordinates numerically. An eigenvalue problem for the system of four ordinary equations is then solved to compute the eigenfrequency and polarization of the standing Alfvén wave for a given field line. As examples, dipole and Tsyganenko 96 magnetic field models are used. The results are compared to the predictions of older models based on either dipole or axisymmetric magnetic fields.

Unusual strong quasi-monochromatic ground Pc5 geomagnetic pulsations in the recovery phase of November 2003 superstorm

Abstract. Unusually large-amplitude morning Pc5 magnetic pulsations during the recovery phase of the huge magnetic storm in November 2003 have been studied by using ground-based multi-point observations. Two main spectral Pc5 enhancements were observed: at f≈2 mHz, which featured slowly increasing frequency with decreasing latitude, and at f≈3 mHz, which was latitude independent. The Pc5 pulsations were observed at wide range of latitudes (more than 10°) with the same very strong amplitude (up to 500nT) and with the same polarization. Only the 3-mHz peak was clearly seen in the spectra of pulsating auroral radio absorption, as observed by the Finnish riometer chain. Short and localized bursts of PiB (f~50–100 mHz) magnetic pulsations and simultaneous short bursts of energetic electron precipitation were observed in the morning sector, as well. The beginning of the large-amplitude morning Pc5 activity occurred simultaneously with a substorm onset in the evening and midnight sectors. However, the spectra of pulsations in the morning and evening sectors were different. They were compared with spectra of IMF and solar wind parameters, measured by ACE spacecraft. The similarity between the spectra of morning Pc5 and IMF By was found, but the spectra of evening Pi3 pulsations were similar to the spectra of solar wind density variations. The Pc5 and PiB pulsations, as well as bursts of the auroral radio absorption, suddenly disappeared, when the solar wind density abruptly dropped. We suppose that the ~2-mHz Pc5 geomagnetic pulsations could be attributed to field line resonance (FLR), however, the 3-mHz oscillations were apparently non-resonance origin. Keywords. Magnetospheric physics (MHD waves and instabilities; Solar wind-magnetosphere interaction; Storms and substorms)

Magnetosphere–ionosphere coupling by Alfvén waves: Beyond current continuity

Many numerical models of magnetospheric dynamics treat the ionosphere as an inner boundary condition. These models have traditionally used the current continuity condition, treating the ionosphere as a sheet current in which the electric field is electrostatic. A more general boundary condition is suggested that not only is more complete, but also straightforward to implement. Results from a model using this boundary condition applied to the excitation of field line resonances in the magnetosphere are presented.

Geomagnetic pulsations caused by the Sumatra earthquake on December 26, 2004

A long period Pc5 pulsation was observed at Phimai in Thailand, shortly after the origin time of the Sumatra earthquake on December 26, 2004. The localized nature and the period of oscillations suggest that the long period magnetic pulsation was generated by dynamo action in the lower ionosphere, set up by an atmospheric pressure pulse which propagated vertically as an acoustic wave when the ocean floor suddenly moved vertically. It is speculated that a Pc3 type pulsation observed at Tong Hai in China, 10 degrees north of Phimai in latitude, was the result of magnetic field line resonance with a magneto‐sonic wave generated from the electric and magnetic fields of the dynamo current caused by the Earthquake.

Travel‐time magnetoseismology: Magnetospheric sounding by timing the tremors in space

This letter introduces the idea of timing the arrival of magnetic impulsive signals at different locations to infer the density distribution of magnetospheric plasma. Tamao's model of magnetohydrodynamic (MHD) wave propagation is used as the underlying principle of this method. The methodology is similar to timing the arrival of seismic signals to understand the Earth's interior. A test of this idea on a sudden impulse event shows that the results are in good agreement with an empirical plasma density model as well as with the density estimated independently by the field line resonance method.

Magnetic Pulsations: Their Sources and Relation to Solar Wind and Geomagnetic Activity

Ultra low frequency (ULF) waves incident on the Earth are produced by processes in the magnetosphere and solar wind. These processes produce a wide variety of ULF hydromagnetic wave types that are classified on the ground as either Pi or Pc pulsations (irregular or continuous). Waves of different frequencies and polarizations originate in different regions of the magnetosphere. The location of the projections of these regions onto the Earth depends on the solar wind dynamic pressure and magnetic field. The occurrence of various waves also depends on conditions in the solar wind and in the magnetosphere. Changes in orientation of the interplanetary magnetic field or an increase in solar wind velocity can have dramatic effects on the type of waves seen at a particular location on the Earth. Similarly, the occurrence of a magnetospheric substorm or magnetic storm will affect which waves are seen. The magnetosphere is a resonant cavity and waveguide for waves that either originate within or propagate through the system. These cavities respond to broadband sources by resonating at discrete frequencies. These cavity modes couple to field line resonances that drive currents in the ionosphere. These currents reradiate the energy as electromagnetic waves that propagate to the ground. Because these ionospheric currents are localized in latitude there are very rapid variations in wave phase at the Earth’s surface. Thus it is almost never correct to assume that plane ULF waves are incident on the Earth from outer space. The properties of ULF waves seen at the ground contain information about the processes that generate them and the regions through which they have propagated. The properties also depend on the conductivity of the Earth underneath the observer. Information about the state of the solar wind and the magnetosphere distributed by the NOAA Space Disturbance Forecast Center can be used to help predict when certain types and frequencies of waves will be observed. The study of ULF waves is a very active field of space research and much has yet to be learned about the processes that generate these waves.

A case study testing the cavity mode model of the magnetosphere

Abstract. Based on a case study we test the cavity mode model of the magnetosphere, looking for eigenfrequencies via multi-satellite and multi-instrument measurements. Geotail and ACE provide information on the interplanetary medium that dictates the input parameters of the system; the four Cluster satellites monitor the magnetopause surface waves; the POLAR (L=9.4) and LANL 97A (L=6.6) satellites reveal two in-situ monochromatic field line resonances (FLRs) with T=6 and 2.5 min, respectively; and the IMAGE ground magnetometers demonstrate latitude dependent delays in signature arrival times, as inferred by Sarafopoulos (2004b). Similar dispersive structures showing systematic delays are also extensively scrutinized by Sarafopoulos (2005) and interpreted as tightly associated with the so-called pseudo-FLRs, which show almost the same observational characteristics with an authentic FLR. In particular for this episode, successive solar wind pressure pulses produce recurring ionosphere twin vortex Hall currents which are identified on the ground as pseudo-FLRs. The BJN ground magnetometer records the pseudo-FLR (alike with the other IMAGE station responses) associated with an intense power spectral density ranging from 8 to 12 min and, in addition, two discrete resonant lines with T=3.5 and 7 min. In this case study, even though the magnetosphere is evidently affected by a broad-band compressional wave originated upstream of the bow shock, nevertheless, we do not identify any cavity mode oscillation within the magnetosphere. We fail, also, to identify any of the cavity mode frequencies proposed by Samson (1992). Keywords. Magnetospheric physics (Magnetosphereionosphere interactions; Solar wind-magnetosphere interactions; MHD waves and instabilities)

Magnetospheric field‐line resonances: Ground‐based observations and modeling

We present theory and ground‐based observations of field‐line resonances (FLRs) excited in Earth's magnetosphere. Three FLR observations are reported, which correspond to large‐scale standing shear Alfvén wave (SAW) oscillations on nightside field lines extending from premidnight to close to dawn. The eigenfrequencies for these events are modeled using a nonorthogonal covariant‐contravariant analysis of the ideal magnetohydrodynamic (MHD) equations. This allows us to use a general field‐line topology, an example of which is computed using the Tsyganenko 1996 magnetic field model. We show that field‐line stretching, along with assumptions regarding the distribution of density along field lines, is sufficient to explain the observed FLR frequencies. Then, we consider dispersive effects operating at the level of the electron inertial scale near the ionosphere or the ion‐acoustic gyroradius in the vicinity of the equatorial plane. Specifically, we estimate the spatial saturation widths and phase mixing timescales based on a simple model of dispersive SAWs. By considering a new model for ionospheric Pedersen conductivity modification by field‐aligned currents in SAWs and numerical (finite element model) solutions to the reduced MHD equations, we model specific FLR observations in an approximate axisymmetric field topology. We discuss the interplay of linear and nonlinear phase mixing, along with gradients in dispersion and time‐dependent losses due to Pedersen conductivity enhancements.

Effects on SuperDARN HF radar echoes of sudden impulses of solar wind dynamic pressure

Abstract. In this work we perform a statistical analysis of the ionospheric echo response observed by six radars of the SuperDARN network in the Northern Hemisphere, over 236 Sudden Impulses (SI) of solar wind dynamic pressure events (from 1997 through 2000). For that purpose, we make use of MRS, the Mean Rate of Scattering, as a function of time during the SI event. We classify the events in sudden increases (I events, 144 cases) and decreases (D events, 92 cases) of the solar wind dynamic pressure. Moreover, we make use of the AE index to define two distinct conditions of the ionosphere under which each event may take place: Quiet and Disturbed. Regarding Quiet conditions, for both I and D events, we find that MRS displays an increase related to the SI time. On the contrary, for Disturbed conditions, D events display an increase in MRS, while I events show a clear dip. The similarity of response for I and D events under Quiet conditions is briefly discussed, but the smaller number of D events does not allow one to further analyse them. As for the I events, a latitudinal analysis shows that the MRS increase for Quiet conditions is seen both at low latitudes (60°–70° Λ) and at high latitudes (70°–80° Λ); for Disturbed Is the MRS decrease is stronger at high latitudes. We suggest that the MRS increase for Quiet Is can be due to two different mechanisms: 1) a soft electron precipitation induced by Field Line Resonances (FLR) or loss cone instability at lower latitudes; 2) an enlargement of the cusp at higher latitudes, which in turn may induce enhanced particle precipitation. For what concerns Disturbed Is, the MRS decrease can be produced by a higher energy electron precipitation (>1 keV), that enhances the electron density in the E and D regions. This provokes a strong absorbtion of the radio waves in the D region and a higher refraction in the E region, leading to a decrease in MRS, especially at higher latitudes. For I events a further classification has been made on the basis of IMF orientation: this suggests that the effects summarized above are due to the SI itself. Keywords. Ionosphere (Ionospheric irregularities) – Magnetospheric physics (Magnetosphere-ionosphere interactions; Solar wind-magnetosphere interactions)

Pc5 modulation of high energy electron precipitation: particle interaction regions and scattering efficiency

Abstract. Using the NORSTAR riometer and CANOPUS magnetometer arrays we have investigated the modulation of high energy electron precipitation by ULF waves in the Pc5 frequency band. We conducted two separate studies of Pc5 activity in the riometers. The first is an independent survey of three riometer stations in the Churchill line (one at each sub-auroral, auroral, and typical polar cap boundary latitudes) in which we identified all riometer Pc5-band pulsations over 11 years. All had a corresponding magnetometer pulsation implying that a magnetic pulsation, is a necessary condition for a riometer pulsation (in the Pc5 Band). We find seasonal and latitude dependencies in the occurrence of riometer pulsations. By a factor of two, there are more riometer pulsations occurring in the fall-winter than the spring-summer. At higher latitudes there is a tendency towards noon pulsations during the spring-summer, suggesting that the criteria for riometer pulsations is affected by the dipole tilt. Our second study was based on the previous magnetometer study of Baker et al. (2003). Using the database of Pc5 activity from that study we were able to select the riometer Pc5 pulsations which adhere to the strict Pc5 definition in the magnetometer. We find that roughly 95% of the riometer pulsations occurred in the morning sector compared to 70% in the magnetometer. Given a magnetometer pulsation at Gillam in the morning sector, there is a 70% chance of there being a corresponding riometer pulsation. The morning sector probabilities at Rankin (geomagnetic (PACE) latitude 74°) and Pinawa (61°) are 3% and 5%, respectively. These statistics suggest there is a localized region in the pre-noon magnetosphere where Pc5 band ULF activity can modulate high energy electron precipitation. We also find that riometer pulsations display a Kp selection towards mid (i.e. 3–4) activity levels which mimics the product of the Kp dependence of high-energy electron fluxes on the dawn side (from CRRES) and all magnetic Pc5 activity. A superposed epoch analysis revealed that the elevated electron flux needed to produce a riometer pulsation is most likely provided by substorm injections on the nightside. We also find that the amplitude of modulated precipitation correlates well with the product of the background absorption and the magnetic pulsation amplitude, again leading to the idea that a riometer pulsation needs both favorable magnetospheric electron flux conditions and large enough magnetic Pc5 wave activity. We further separate our pulsations into field line resonances (FLRs), and non-field line resonances (non-FLRs), as identified in the Baker et al. (2003) survey. We find that FLRs are more efficient at modulating particle precipitation, and non-FLRs display an amplitude cutoff below which they do not interact with the high energy electron population. We conclude that the high energy electron precipitation associated with Pc5 pulsations is caused by pitch angle scattering (diffusion) rather than parallel acceleration. We suggest two future studies that are natural extensions of this one. Keywords. Energetic Particles/Precipitating; Wave-Particle Interactions; Auroral Phenomena

Resonant absorption of an Alfvén wave: Hybrid simulations

We examine the evolution of a planar Alfvén wave propagating along and across a pressure‐balanced structure. The structure has mostly a magnetic field component perpendicular to a uniform background magnetic field B0 and gives an almost sinusoidal Alfvén speed gradient across B0. The Alfvén wave propagates both oblique to the structure and B0. A hybrid simulation code with particle protons and massless fluid electrons is used to follow evolution. The structure advects little with the wave so that the wave evolves mostly in accord with a fixed Alfvén speed gradient. The Alfvén wave undergoes resonant absorption wherein one set of resonant field lines loses wave energy to another set of resonant field lines by cross‐field energy transport. We derive the timescale of the cross‐field transfer and associate it with global modes. This feature of wave evolution occurs independently of ion kinetics. On resonant field lines, wave refraction generates ever smaller scales perpendicular to B0. When the perpendicular scales are of order 10 proton inertial lengths (10c/ωpi), a nonpropagating density fluctuation begins to grow to large amplitudes. The growth saturates by exciting highly oblique, compressive, and low‐frequency waves which dissipate and heat protons. Protons are heated approximately by equal amounts along and across the local magnetic field. Heating continues until most of the resonantly absorbed Alfvén wave energy is exhausted. This process limits density fluctuations to scales across B0, which are greater than ≈ c/ωpi. This process could play a role in the solar corona where a cutoff of density fluctuations at scales near and below c/ωpi is inferred from radio observations.

Pc3-4 ULF waves observed by the SuperDARN TIGER radar

Abstract. Despite extensive research, the mechanisms for propagation of Pc3-4 energy from the generation region at the bow shock to the high-latitude ionosphere remain unresolved. We used high temporal (6-12s) and spatial (45km) resolution data from the SuperDARN TIGER radar (Tasmania) to examine Pc3-4 wave signatures at the F-region heights. We focus on a case study on 28 September 2000, when large-amplitude band-limited Pc3-4 oscillations were observed across 10-20 range gates in beam #4 (which points towards the CGM pole) for about four hours preceding MLT noon. These waves were detected in sea-scatter echoes reflected from the ionospheric footprint of the plasmatrough. Nearby ground magnetometer data from Macquarie Island showed very similar variations in both the north-south and east-west components. The radar data revealed the occasional presence of quasi-FLR (field-line resonance) spatial structures with frequencies much higher than those of the local fundamental FLR modes. Detailed spectral analysis of the ionospheric and ground data shows that these structures most probably correspond to a 3rd-harmonic, poloidal-mode FLR. Such observations suggest that compressional Pc3-4 waves produced in the upstream solar wind travel earthward from the magnetopause in the magnetic equatorial plane depositing energy into the Alfvenic modes, as either forced or 3rd-harmonic FLR that reach ionospheric heights along magnetic field lines.

Quasi-periodic oscillations: Resonant shear Alfvén waves in neutron star magnetospheres

We propose a new model for quasi-periodic oscillations (QPOs) based on oscillating magnetohydrodynamic modes in neutron star magnetospheres. We argue that the interaction of the accretion disk with the magnetosphere can excite resonant shear Alfvén waves in a region of enhanced density gradients, the region where accretion material flows along the magnetic field lines in the magnetosphere (see Fig.1). We demonstrate that depending on the distance of this region from the star and the magnetic field strength, the frequency of the field line resonance can range from several Hz (a weaker field, farther from the star), to approximately kHz frequencies (a stronger field, ~ stellar radii from the star). We show that such oscillations are able to significantly modulate inflow of matter from the high density region toward the star's surface and possibly produce the observed X-ray spectrum. In addition, we show that the observed 2:3 frequency ratio of QPOs is a natural result of our model.